1. Field of the Invention
The invention relates to a metallocene catalyst useful in preparing polyolefins having low molecular weight and narrow molecular weight distribution. The catalyst comprises a bridged metallocene having two cyclopentadienyl rings, one substituted with a bulky group in a distal position and one substituted such that a fused ring is formed. The invention further includes a process of preparing polyolefins having low molecular weight and narrow molecular weight distribution using the disclosed catalysts. The polymer product can be used as a crystalline wax or a functionalizable macromer formed of a olefin terminated chain.
2. Description of the Prior Art
The present invention provides a process for polymerizing olefins to produce a polymer with a low molecular weight and a narrow molecular weight distribution. The catalyst and process are particularly useful in polymerizing ethylene and propylene to form crystalline waxes or functionalizable macromers having low molecular weight and narrow molecular weight distribution.
Polypropylene may vary by stereochemical structure. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: 
Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: 
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each xe2x80x9crxe2x80x9d represents a xe2x80x9cracemicxe2x80x9d dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene.
Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product.
Catalysts that produce isotactic polyolefins are disclosed in European Patent Application Publication No. 284,708 corresponding to U.S. patent application Ser. No. 034,472 filed Apr. 3, 1987, and now abandoned; U.S. Pat. No. 4,794,096; and European Patent Application Publication No. 310,734 corresponding to U.S. patent application Ser. No. 095,755 filed on Sep. 11, 1987, and now abandoned. These applications disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of a highly isotactic polypropylene. The present invention, however, provides a different class of metallocene catalysts that are useful in the polymerization of olefins, and more particularly, ethylene and propylene, to produce a crystalline wax.
The present invention provides a catalyst and process for preparing crystalline polyolefins having low molecular weight, and more particularly, polyethylene and polypropylene having low molecular weights.
The catalyst comprises a metallocene, i.e., a metal derivative of a cyclopentadiene, and an ionizing agent. The metallocene compound should contain two cyclopentadienyl rings and be of the general formula:
Rxe2x80x3(C5R4)(R2C5C4Rxe2x80x24)MEQp
wherein (C5R4) is substituted cyclopentadienyl ring; (R2C5C4Rxe2x80x2n) is an indenyl or substituted indenyl ring; each R and Rxe2x80x2 is hydrogen or hydrocarbyl radical having from 1-20 carbon atoms, a halogen, an alkoxy, and alkoxy alkyl or an alkylamino radical, each R and Rxe2x80x2 may be the same or different; (C5R4) has one R substituent in a distal position which is at least as bulky as a t-butyl radical; Rxe2x80x3 is a structural bridge between the (C5R4) and (R2C5C4Rxe2x80x24) rings to impart stereorigidity and is preferably a hydrocarbyl or hydrosilyl radical having one atom of carbon or silicon to form the bridge; Q is a hydrocarbyl radical, such as an alkyl, aryl, alkenyl, alkylaryl or arylalkyl radical having 1-20 carbon atoms or is a halogen; Me is a Group IIIB, IVB, VB, or VIB metal as positioned in the Periodic Table of Elements; and p is the valence of Me minus 2.
The present invention further provides a process for producing polyolefins, particularly atactic polypropylene of low tacticity, i.e., neither predominantly isotactic or syndiotactic, having low molecular weight and narrow molecular weight distribution. The process comprises utilizing at least one of the catalysts described by the above formula and introducing the catalyst into a polymerization reaction zone containing an olefin monomer. In addition, an ionizing agent or cocatalyst such as alumoxane may be introduced into the reaction zone. Further, the catalyst may also be pre-polymerized prior to introducing it into the reaction zone and/or prior to the stabilization of reaction conditions in the reactor.